1. Field of the Invention
The present invention relates generally to passive microwave devices, and particularly to microwave circulator/isolator devices.
2. Technical Background
A ferrite circulator/isolator is a passive multi-port microwave device that is typically employed in RF transmission line applications such as radar, cell phone applications, etc. The ferrite circulator/isolator device is typically used to provide a low loss transmission path for RF energy in one direction and substantially prevent any transmission of energy in the reverse direction. If a reflected RF signal or some other RF signal is permitted to propagate in the reverse direction, an unprotected signal source may be significantly damaged. The ferrite circulator/isolator device is configured to attenuate such RF transmissions to thereby prevent such damage from occurring.
A typical ferrite circulator includes three outgoing ports, and is generally referred to as a junction circulator. In operation, when an RF signal is directed into a first port, the RF signal will be accessible via the second port in sequence, i.e., the port immediately adjacent to the input port. Accordingly, the RF signal will be substantially attenuated and will not be available at the third port in the sequence, that is, the port immediately adjacent to the second port on the other side of the first input port. A circulator, therefore, propagates RF power from one adjacent port to the next in a sequential, circular fashion. The RF signal circulation may be right-handed (RH) or left-handed (LH).
When an RF signal is directed into the input port of the circulator, circulating phase shifted versions of the RF signal are induced within the ferrite discs. At an operation frequency range the degree of phase shift between counter circulating fields is a function of the strength of the DC magnetic field and diameter of the ferrite material. The circulator operates in accordance with the principles of superposition and constructive/destructive interference of counter-rotating RF waves. Using the example from above, when an RF signal is directed into the first port, the counter circulating RF signals are substantially in phase with each other at the second port, and therefore, they constructively interfere and reinforce each other. The amount of signal available at the second port as compared with the input signal is measured by what is commonly referred to as the insertion loss. At the third port, the RF signals are out of phase with each other and substantially cancel each other. The term “substantially” refers to the fact that, in practice, the cancellation is not perfect and a residual signal may be detected. The amount of residual signal available at the third port is measured by the degree of isolation (dB).
Insertion loss is so named because it represents the loss of signal power associated with inserting the device into the signal path. In a properly functioning device the insertion loss is typically in the range of a few tenths of a decibel (dB). At the third port, the RF signals are out of phase with each other and substantially cancel each other. As alluded to above, the amount of residual signal available at the third port, appropriately referred to as the “isolation,” is measured by the ratio of the residual signal and the incident signal. The isolation is typically between −25 dB and −30 dB. Another parameter frequently discussed in the art is known as return loss. Return loss relates to the amount of signal power that is reflected when the device is inserted into the signal path. Return loss is also typically expressed in decibels (dB). As those of ordinary skill in the art will appreciate, signal reflections are introduced when there is an impedance mismatch at the junction of two signal paths.
A junction circulator includes both electrical and magnetic circuit components and may be implemented using either a stripline or microstrip transmission line configuration. The first sub-assembly discussed herein is referred to as the central stack assembly. The electrical portion of the central stack includes a flat conductor, circuit junction. The circuit junction is sandwiched between a pair of ferrite discs. The outer surface of both the top ferrite disc and bottom ferrite disc are in contact with ground planes to thereby form a stripline configuration. A permanent magnet is disposed over each ground plane. The permanent magnets apply a predetermined magnetic field to bias the ferrite discs normal to plane. A steel pole member may be inserted between each ground plane/magnet pair. The function of the steel pole member is to ensure that the biasing magnetic field applied to the ferrites is substantially uniform. The magnetic properties of both the ferrite material and the magnet may result in temperature variations. Therefore, the central stack may also include thermal compensators that are configured to ensure that the thermal stability of the circulator is maintained. The thermal compensators, which may be fabricated using nickel alloys, offset the aforementioned temperature variations.
A circulator may be configured as an isolator by terminating one of the ports with a “matched load.” In implementing a matched load, RF engineers ensure that, from an impedance standpoint, the complex impedance of the load is the complex conjugate of the output port impedance. As noted above, an isolator permits RF signal propagation between the two remaining ports in one direction only. RF power flow in the opposite direction is substantially inhibited. Now that the general operating principles have been briefly touched upon, a similarly brief description of the matching structure of a junction circulator is provided.
The circuit junction is composed of a center resonator and three branches extending symmetrically outward from the central conductive portion. The three branches known in the art as impedance transformers function as the ports of circulator and are positioned 120° apart from each other. It is known that the impedance of the center resonator itself is usually well below of 50 Ohms. At the same time, the circulators and isolators are utilized in the systems with characteristic impedance of 50 Ohms. The transformation of low impedance at center resonator to 50 Ohms at ports is usually realized by appropriately designing the geometry of outgoing transformer branches. In Wye-type circulators the center resonator and outgoing transformer sections are formed to fit within the area covered by ferrite discs. Incorporating transformer section within ferrite area allows for a substantial reduction in the size of circulator. Generally all three circulator ports have the same impedance value. Therefore, the symmetry considerations require the transformer branches to be the same for all ports and the center resonator to be aligned with the symmetry axis of ferrite discs. In some applications, like an isolator with an external load, the impedance of a device connected to the circulator may deviate from 50 Ohms. If the deviation is small, the matching is realized by modifying the appropriate transformer section to bring the impedance of the particular port to match that of connected device. If, however, the impedance of device deviates substantially from 50 Ohms, as is a case of high power transistor, the aforementioned modification does not typically provide a suitable result. The use of additional external transformers has drawbacks because the external transformers tend to increase the size of device and introduce additional losses of power.
What is needed, therefore, is a circulator/isolator that is designed having an internal feature that provides matching in case when the impedance requirement for one port is substantially different than that for other two ports.